A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape. The most common porphyroblasts in metapelites (metamorphosed mudstones and siltstones) are garnets and staurolites, which stand
Found on http://en.wikipedia.org/wiki/Porphyroblast
(from the article `metamorphic rock`) ...crystals showing planar surfacesnamely, magnetite, garnet, epidote, mica, calcite, quartz, and feldspar. Minerals that have a tendency to form ...
Found on http://www.britannica.com/eb/a-z/p/95
A large crystal of a mineral such as garnet or staurolite set in a matrix of much finer-grained minerals in a metamorphic rock. compare phenocryst.
Found on http://www.ge-at.iastate.edu/glossary-of-geologic-terms/
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